The generation of a magnetic latent image on a magnetizable layer coated on a supporting base is well known in the art. The first demonstration is believed to date back as far as 1839 to the experiments of A. W. Jones. These involved creating a latent magnetic image by writing on a blackened iron plate with a magnetized stylus.
Thermal magnetography, as a process of producing a latent image on a ferromagnetic layer through the use of selective imagewise heating of the layer, has since become quite widely known. However, in common with most nonsilver halide imaging systems, this process suffers from low photographic speed. That is, a substantial amount of energy is required to selectively change the property of selected portions of the imaged surface to provide an adequate signal that can be used to read out the changed portions.
When using thermal magnetography to create the latent magnetic image, an alternating magnetic pattern is typically impressed on the surface by one of several methods with from about 50 to 4000 magnetic flux reversals per inch (19.7 to 1574.8 per cm) and preferably from 100 to 3000 magnetic flux reversals per inch (39.37 to 1181.1 per cm). It should be noted that one cycle per inch is comprised of two flux reversals per inch.
Frequently, the surface of the magnetic layer is thermoremanently structured by placing the magnetic member having a continuously coated surface of magnetic material on top of a magnetic master recording of the desired periodic pattern. An external energy source is then applied to heat the surface of the magnetic member above its Curie temperature. As the surface of the magnetic member cools below its Curie temperature, it is thermoremanently magnetized by the periodic magnetic signal from the magnetic master recording. When acicular chromium dioxide is used as the magnetic material in the surface of the magnetic member, as little as 25 oersteds can be used to structure the surface of the magnetic member when passing through the Curie temperature, whereas over 200 oersteds are needed to impose detectable magnetism to acicular chromium dioxide at room temperature.
Alternatively a magnetic recording head extending the full width of the magnetic layer may be used to impose an alternating magnetic pattern on the layer in much the same way as sound recordings are made on a moving magnetic tape.
As early as 1971, G. R. Nacci, in U.S. Pat. No. 3,555,556 disclosed the basic process of thermomagnetic recording and further disclosed that, in the process of demagnetizing imagewise a premagnetized magnetic layer comprising acicular thermomagnetic particles in a binder coated on a nonmagnetic support surface, the demagnetization does not need to be carried out completely throughout the thickness of the layer, but that adequate imaging may be obtained even with partial depthwise demagnetization of the layer. An important application of the use of thermal magnetography is shown in U.S. patent application Ser. No. 173,871, filed July 30, 1980 and now U.S. Pat. No. 4,338,391. In this application, a latent magnetic image is read out by decorating it with toner particles which are then transferred under pressure to a heated substrate.
As the use of thermal magnetography becomes more widespread, it is, of course, increasingly desirable that the latent magnetic image necessary for use in many of those applications be produced in the most energy efficient manner in order to permit higher image generation speeds and to incur less power waste. In practical applications where the magnetic layer contains a resin binder, the lower energy input results in less heating of the binder, which, in turn, eliminates some of the troublesome problems such as thermal distortion and degradation of the resin binder.